1. Field of the Invention
This invention relates to a multi-piece solid golf ball having a cover of inner and outer layers and more particularly, to a multi-piece solid golf ball in which cover hardness and dimples are optimized to improve flight distance performance.
2. Prior Art
Golf balls are generally classified into solid golf balls in which a solid core is enclosed with at least one layer of cover and wound golf balls in which a wound core in the form of a center ball having thread rubber wound thereon is enclosed with a cover. Numerous modifications were heretofore proposed to improve flight distance properties, spin performance, and controllability.
As one example of such proposals, an approach of increasing a spin rate by forming the cover soft or to low hardness falls under the category of the prior art. In particular, improvements in multi-piece solid golf balls are by adjusting the composition and hardness of the thermoplastic resin of which each cover layer is constructed. For example, if it is desired to increase a spin rate, the outer cover layer coming in direct contact with the club face is formed relatively soft in consideration of a friction phenomenon upon impact. Inversely, if it is desired to decrease a spin rate, the outer cover layer is formed relatively hard.
However, the multi-piece solid golf balls wherein the outer cover layer is formed relatively soft have the problem that a desired spin rate is not always obtained because the hardness of the inner cover layer in contact with the outer cover layer is not optimized. Therefore, the deformation process upon impact differs among the respective layers.
Also proposed were techniques of forming the inner cover layer relatively soft in order to increase a spin rate and forming the outer and inner cover layers relatively soft in order to further increase a spin rate. There arises the problem that the trajectory changes in flight to adversely affect the flight distance.
On the other hand, for those golf balls required to have flight distance performance, it is difficult to form dimples suitable for the spin range and restitution which vary with the cover hardness. Golf balls with dimples of one type suffer from the problem that they rise too high or drop to detract from flight distance performance.
The present invention has been made under the above-mentioned circumstances and its object is to provide a golf ball comprising a solid core enclosed with two inner and outer layers enabling an increase of flight distance.
Making extensive investigations to achieve the above object, the inventors have found in connection with a multi-piece solid golf ball comprising a solid core and a cover of two inner and outer layers surrounding the core, the outer cover layer being formed in the surface with a plurality of dimples, that a spin rate is approximately explained in terms of a product of the Shore D hardnesses of the inner cover layer multiplied by the Shore D hardnesses of the outer cover layer. More particularly, a higher spin rate is obtained when the product of the Shore D hardnesses of the inner and outer layers has a relatively smaller value. Inversely, a reduced spin rate is obtained when the same product has relatively larger value. Accordingly, one effective means for taking full advantage of the spin property dependent on the product of the Shore D hardnesses of the inner and outer layers and improving the flight performance of the golf ball is to divide the range of the product into sub-ranges and form dimples so as to satisfy the following two requirements associated with the sub-ranges of the product. More particularly, it has been found effective as a first requirement to specify a proportion VR (%) of the total of the volumes of dimple spaces each defined below a plane circumscribed by the dimple edge to the overall volume of a phantom sphere given on the assumption that the golf ball surface is free of dimples. A second requirement is to form at least three types of dimples which are different in at least one of a diameter, a depth, and a value V0 which is the volume of one dimple space defined below a plane circumscribed by the dimple edge divided by the volume of a cylinder whose bottom is the plane and whose height is the maximum depth of the dimple from the bottom. The inventors have also found that to specify the distortion of the solid core and to specify the Shore D hardnesses of the inner and outer cover layers are more effective. The present invention is predicated on this finding.
Specifically, the present invention provides:
1) A multi-piece solid golf ball comprising a solid core and a cover of two inner and outer layers surrounding the core, the outer cover layer having a surface formed with a plurality of dimples, characterized in that
a product of the Shore D hardnesses of said inner cover layer multiplied by the Shore D hardnesses of said outer cover layer and a proportion VR (%) of the total of the volumes of dimple spaces each defined below a plane circumscribed by the dimple edge to the overall volume of a phantom sphere given on the assumption that the golf ball surface is free of dimples satisfy any one of the following combinations (1) to (5):
(1).the product of Shore D hardnesses of inner and outer cover layers: 1,500 to less than 2,000
VR: 0.8 to 1.1%
(2) the product of Shore D hardnesses of inner and outer cover layers: 2,000 to less than 2,500
VR: 0.75 to 1.05%
(3) the product of Shore D hardnesses of inner and outer cover layers: 2,500 to less than 3,000
VR: 0.7 to 1%
(4) the product of Shore D hardnesses of inner and outer cover layers: 3,000 to less than 3,500
VR: 0.65 to 0.95%
(5) the product of Shore D hardnesses of inner and outer cover layers: 3,500 to 4,000
VR: 0.6 to 0.9%,
and said dimples include at least three types of dimples which are different in at least one of a diameter, a depth, and a value V0 which is the volume of one dimple space defined below a plane circumscribed by the dimple edge divided by the volume of a cylinder whose bottom is the plane and whose height is the maximum depth of the dimple from the bottom.
2) The multi-piece solid golf ball of 1) wherein the solid core has a distortion of 2.6 to 6.5 mm under an applied load of 100 kg.
3) The multi-piece solid golf ball of 1) or 2) wherein both the hardnesses of the inner and outer cover layers are up to 63 in Shore D hardness.